Moved MAX_CONCURRENT_IO_REQUESTS to pool.cc.  Removed TARGET_IO_QUEUE_DEPTH.
Made pool_diskmgr_t take max_concurrent_io_requests as a runtime parameter.
Made --io-load command line opton to specify io load limit.
Fixed naming of --io-load-limit.
Made io-load-limit option not default to the string "DEFAULT_MAX_CONCURRENT_IO_REQUESTS" and instead default to its value.
Renamed --io-load-limit to --max-io-threads.
Merge branch 'next' into sam_928
Made block_pool_queue_depth use MAXIMUM_MAX_CONCURRENT_IO_REQUESTS instead of MILLION.
Renamed --max-io-threads to --io-threads.
Merge branch 'next' into sam_928
Factored out --cores and --io-threads parsing/erroring logic.